arXiv:astro-ph/0211108 v2 17 Oct 2003
Bull. Astr. Soc. India (2003) 31, 000–000
Optical observations of the bright long duration
peculiar GRB 021004 afterglow
S.B. Pandey1 , D.K. Sahu2,3 , L. Resmi4,5 , R. Sagar1,3, G.C. Anupama3 ,
D. Bhattacharya4 , V. Mohan1 , T.P. Prabhu3 , B.C. Bhatt2,3 , J.C. Pandey1 ,
Padmaker Parihar2,3 and A.J. Castro-Tirado6
1
2
3
4
5
6
State Observatory, Manora Peak, Nainital – 263 129, India
Center for Research & Education in Science & Technology, Hosakote, Bangalore – 562 114, India
Indian Institute of Astrophysics, Bangalore – 560 034, India
Raman Research Institute, Bangalore – 560 080, India
Joint Astronomy Programme, Indian Institute of Science, Bangalore – 560 012, India
Instituto de Astrofísica de Andalucía, P.O. Box 03004, E-18080, Granada, Spain
Received 2002 November 6; accepted 2003 February 10
Abstract.
The CCD magnitudes in Johnson B, V and Cousins R and I
photometric passbands are determined for the bright long duration GRB 021004
afterglow from 2002 October 4 to 16 starting ∼ 3 hours after the γ−ray burst.
Light curves of the afterglow emission in B,V ,R and I passbands are obtained by
combining these measurements with other published data. The earliest optical
emission appears to originate in a revese shock. Flux decay of the afterglow
shows a very uncommon variation relative to other well-observed GRBs. Rapid
light variations, especially during early times (∆t < 2 days) is superposed on
an underlying broken power law decay typical of a jetted afterglow. The flux
decay constants at early and late times derived from least square fits to the
light curve are 0.99 ± 0.05 and 2.0 ± 0.2 respectively, with a jet break at around
7 day. Comparison with a standard fireball model indicates a total extinction
of E(B − V ) = 0.20 mag in the direction of the burst. Our low-resolution
spectra corrected for this extinction provide a spectral slope β = 0.6 ± 0.02.
This value and the flux decay constants agree well with the electron energy
index p ∼ 2.27 used in the model. The derived jet opening angle of about 7◦
implies a total emitted gamma-ray energy Eγ = 3.5 × 1050 erg at a cosmological
distance of about 20 Gpc. Multiwavelength observations indicate association of
this GRB with a star forming region, supporting the case for collapsar origin
of long duration GRBs.
2
Pandey et al.
Keywords : Photometry – spectroscopy – GRB afterglow – flux decay – spectral
index
1.
Introduction
In recent years, both photometric and spectroscopic optical observations of Gamma-Ray
Burst (GRB) afterglows have provided valuable information about the emission from
GRBs. While spectral lines have been used to determine redshift distances and to study
the host galaxies, photometric light curves have unravelled the physical parameters and
dynamical evolution of GRB afterglows (cf. Panaitescu & Kumar 2002, Sagar 2001, 2002
and references therein).
A long duration GRB 021004 (≡ H2380) triggered at 12h 06m 13.s 57 UT on 4 October
2002 was detected by the HETE FREGATE, WXM, and soft X−ray camera (SXC)
instruments (Shirasaki et al. 2002). The burst had a duration of ∼ 100 seconds in both
FREGATE 8-40 Kev and WXM 2-25 Kev bands. Analyses of the FREGATE and WXM
data by Lamb et al. (2002) show that the spectrum of the burst is well characterized by a
single power-law with slope 1.64±0.09. The burst fluences are 0.75, 1.8 and 3.2 µerg/cm2
in the energy bands of 2 – 25 Kev, 50 – 300 Kev and 7 – 400 Kev respectively. The
fluence ratio S(2-25)/S(50-300) = 0.42 indicates that it is an X−ray rich GRB.
The SXC coordinates of the burst reported by Doty et al. (2002) are α = 00h 26m 55.s 75, δ =
′
′′
+18◦ 56 18. 6 (J2000). A relatively bright with R ∼ 15.5 mag optical afterglow (OA)
of the GRB 021004 was discovered by Fox (2002) about 9 minutes after the burst at
′
′′
′′
α = 00h 26m 54.s 687, δ = +18◦55 41. 3 (J2000) with an uncertainty of 0. 5 in each coordinates. The astrometric position of the OA determined by Henden & Levine (2002)
′
′′
is α = 00h 26m 54.s 674, δ = +18◦ 55 41. 59 (J2000) with ∼ 50 mas external error in each
coordinates. This is in excellent agreement with the coordinates given by Fox (2002).
Thus, GRB 021004 becomes second burst after GRB 990123 whose OA could be observed within few minutes of the trigger of the event. At the location of OA, Wood-Vasey
et al. (2002) found no source brighter than R ∼ 22 mag on images taken on 3 October
2002 at 07h 24m 18s , 07h 54m 50s and 08h 25m 19s UT while Sako & Harrison (2002) report a
fading X−ray source with a power-law time slope of −1.1 ± 0.1 using the Chandra X−ray
observations taken with the High-Energy Transmission Grating on 2002 October 5, about
20.5 hours after the burst. Almost within a day after the burst, the radio afterglow was
also detected by Frail & Berger (2002) at 22.5 GHz; by Pooley (2002) at 15 GHz and
by Bremer & Castro-Tirado (2002) at 86 GHz. The polarimetric observations taken on
2002 October 05.077 and 08.225 UT indicate almost zero V −band intrinsic polarization
for the OT (Covino et al. 2002; Rol et al. 2002, Wang et al. 2002).
Based on the detection of ionised Mg, Mn and Fe absorption features, Fox et al. (2002)
indicated two redshift values, z = 1.38 and 1.60. Eracleous et al. (2002) and Anupama et
Optical observations of GRB 021004 afterglow
3
al. (2002) also confirm the presence of two absorption systems. Chornock & Filippenko
(2002) identified in addition to them, the emission lines at z = 2.323. The existence of
this multi-component z ∼ 2.3 redshift systems was also confirmed by Castro-Tirado et
al. (2002), Djorgovski et al. (2002), Mirabal et al. (2002b), Salamanca et al. (2002)
and Savaglio et al. (2002) in the high resolution spectrum of the OA. The spectroscopic
variability studied by Matheson et al. (2003) indicates that there is a colour evolution
with the OA becoming redder with time, implying a (B-V) increase of about 0.2 - 0.3
mag over the first three days. The spectrum of the OA consists of a blue continuum with
several absorption features corresponding to two intervening metal-line systems at z =
1.380 and 1.602 and one set of lines at a redshift of z = 2.323, apparently intrinsic to
the host galaxy of the GRB. Møller et al. (2002), on the other hand, identify absorption
lines from five systems at z = 1.3806, 1.6039, 2.2983, 2.3230 and 2.3292 along with an
emission line at z = 2.3351.
There are no photometric standards in the field of GRB 021004 and the photometric
calibrations published after the burst by Weidinger et al. (2002) and Henden (2002) show
a zero-point difference of 0.12 mag in R. A comparison of Henden (2002) photometry
with that of Barsukova et al. (2002) for 3 common star indicates that former is brighter
by 0.2 to 0.3 mag in B; fainter by 0.01 to 0.11 mag in V but agrees within errors in
R. For reliable determination of the OA magnitudes, secured photometric calibrations
are needed. In order to provide them, we imaged the field of GRB 021004 along with
SA 92 standard region of Landolt (1992). A total of 40 secondary stars in the field have
been calibrated and their standard BV RI magnitudes are given here. Our observations
started about 3 hour after the burst and are valuable for dense temporal coverage of the
light curve. We present the details of our optical observations in the next section, and
discuss the optical light curves and other results in the remaining sections.
2.
Optical observations, data reductions and calibrations
The broad band photometric and low-resolution spectroscopic optical observations obtained for the GRB 021004 afterglow are described below along with their data reduction
and calibration.
2.1
Broad band photometric data
The broad band Johnson BV and Cousins RI observations of the OA were carried out
between 4 to 16 October 2002 using 2-m Himalayan Chandra Telescope (HCT) of the
Indian Astronomical Observatory (IAO), Hanle and the 104-cm Sampurnanand telescope
of the State Observatory, Nainital. At Nainital, one pixel of the 2048 × 2048 pixel2 size
′′
CCD chip corresponds to 0. 38 square, and the entire chip covers a field of ∼ 13′ × 13′
on the sky. The gain and read out noise of the CCD camera are 10 e− /ADU and 5.3 e−
′′
respectively. At Hanle, one pixel corresponds to 0. 3 square, and the entire chip covers a
4
Pandey et al.
field of ∼ 10′ × 10′ on the sky, it has a read out noise of 4.95 e− and gain is 1.23 e− /ADU .
From Nainital, the CCD BV RI observations of the OA field along with Landolt (1992)
standard SA 92 region were obtained on 13/14 October 2002 during good photometric
sky conditions for photometric calibration. During the observing run, several twilight flat
field and bias frames were also obtained for the CCD calibrations.
The CCD frames were cleaned using standard procedures. Image processing was done
using ESO MIDAS, NOAO IRAF and DAOPHOT softwares. Atmospheric extinction
coefficients determined from the Nainital observations of SA 92 bright stars are 0.34,
0.22, 0.17 and 0.14 mag in B, V, R and I filters respectively on the night of 13/14 October
2002. They are used in our further analyses. There are nine standard stars in the SA
92 region. They cover a wide range in colour (0.64 < (V − I) < 1.84) as well as in
brightness (12.5 < V < 15.6). The transformation coefficients were determined by fitting
least square linear regressions to the following equations.
bCCD = B − (0.036 ± 0.01)(B − V ) + (5.08 ± 0.02)
vCCD = V − (0.051 ± 0.01)(B − V ) + (4.56 ± 0.01)
rCCD = R − (0.003 ± 0.01)(V − R) + (4.38 ± 0.01)
iCCD = I − (0.026 ± 0.01)(V − R) + (4.87 ± 0.02)
where BV RI are standard magnitudes and vCCD , bCCD , rCCD and iCCD represent the
instrumental magnitudes normalized for 1 second of exposure time and corrected for
atmospheric extinction. The errors in the colour coefficients and zero points are obtained
from the deviation of data points from the linear relation. Using these transformations,
BV RI photometric magnitudes of 40 secondary standard stars are determined in the
GRB 021004 field and their average values are listed in Table 1. The (X, Y ) CCD pixel
coordinates are converted into α2000 , δ2000 values using the astrometric positions given
by Henden (2002). All these stars have been observed 3 to 17 times in a filter and
have internal photometric accuracy better than 0.01 mag. Henden (2002) also provides
the U BV RI photometry for a large number of stars in the field. A comparison in the
sense present minus Henden (2002) value yields small systematic zero-point differences
of −0.002 ± 0.03, 0.013 ± 0.02, 0.02 ± 0.026 and 0.03 ± 0.03 mag in B, V, R and I filters
respectively. These numbers are based on 25 common stars having range in brightness
from V = 14 to 18 mag and can be accounted for in terms of zero-point errors in the
two photometries. There is no colour dependence in the photometric differences. We
therefore conclude that photometric calibration used in this work is secure.
Several short exposures up to a maximum of 15 minutes were generally given while
imaging the OA (see Table 2). In order to improve the signal-to-noise ratio of the OA,
the data have been binned in 2 × 2 pixel2 and also several bias corrected and flat-fielded
CCD images of OA field taken on a night are co-added in the same filter, when found necessary. From these images, profile-fitting magnitudes are determined using DAOPHOT
software. For determining the difference between aperture and profile fitting magnitudes,
we constructed an aperture growth curve of the well isolated stars and used them to
determine aperture (about 5 arcsec) for the magnitudes of the OA. They are calibrated
Optical observations of GRB 021004 afterglow
5
differentially with respect to the secondary standards listed in Table 1 and the values
derived in this way are given in Table 2. They supersede the values published earlier by
Sahu et al. (2002).
The secondary standards are also used for calibrating other photometric measurements of OA published by the time of paper submission by Bersier et al. (2003), Covino
et al. (2002), Di Paola et al. (2002), Fox (2002), Garnavich & Quinn (2002), Holland et
al. (2003), Halpern et al. (2002a, b), Malesani et al. (2002a, b), Masetti et al. (2002),
Matsumoto et al. (2002), Mirabal et al. (2002a, b), Oksanen et al. (2002), Rhoads
et al. (2002), Stefanon et al. (2002), Williams et al. (2002), Winn et al. (2002) and
Zharikov et al. (2002). In order to avoid errors arising due to different photometric
calibrations, we have used only those published BV RI photometric measurements whose
magnitudes could be determined relative to the stars given in Table 1. The JHK magnitudes are adopted from Di Paola et al. (2002), Rhoads et al. (2002) and Stefanon et
al. (2002). The distribution of photometric data points taken from the literature and
from the present measurements are N (U, B, V, R, I, J, H, K) = (6, 25, 31, 197, 23, 2, 3, 3)
and N (B, V, R, I) = (15, 27, 67, 29) respectively. Thus, a total of 428 photometric data
points in eight passbands are there for our analysis in the optical and near-IR region.
2.2
Spectroscopic observations
CCD low-resolution spectra of the OA were obtained, from IAO, on 2002 October 4.789,
4.806, 4.876 and 4.894 UT, using the Hanle Faint Object Spectrograph Camera instrument. The epochs correspond to 0.285, 0.302, 0.372 and 0.39 day respectively after the
burst. The exposure times were 900s for the first two and 1200s for the last two spectra.
They were obtained at a resolution of 18 Å, using a slit width of 2′′ , covering a wavelength
range of 5200–9000 Å. Spectrophotometric standard BD+28◦ 4211 was observed with a
wider slit of 15′′ width.
All spectra were bias subtracted, flat-field corrected, extracted and wavelength calibrated in the standard manner using the IRAF reduction package. The spectra were corrected for instrumental response and brought to a flux scale using the spectrophotometric
standard. Since the position angle of the slit was not along the parallactic angle (Filippenko 1982), and the observations were made at an airmass ∼ 2.2, the fluxes of the OA
have been calibrated using zero points derived from BV RI photometry. The spectra have
been corrected for a total (Galactic and/or host galaxy) extinction of E(B − V ) = 0.20
mag (see section 4 for details) and shown in Fig 1. The spectrum shows a blue continuum
with superposed absorption features. The absorption systems are identified with two intervening metal-line systems at z = 1.38 and 1.60. The line center of the absorption
features, their identification and the inferred redshift values are listed in Table 3. The
line systems are marked in Fig 1. Present results supercede the analysis presented by
Anupama et al. (2002) and agree well with other spectroscopic determinations published
in the literature.
6
Pandey et al.
Table 1. The identification number(ID), (α, δ) for epoch 2000, standard V, (B − V ), (V −
R) and (R − I) photometric magnitudes of the stars in the GRB 021004 region are given.
N(B,V,R,I) denotes the number of observations taken in B, V, R and I filters respectively.
Star 23 is the comparison star mentioned by Henden (2002).
ID
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
α2000
(h m s)
00 26 29
00 26 31
00 26 32
00 26 32
00 26 34
00 26 34
00 26 35
00 26 37
00 26 38
00 26 39
00 26 44
00 26 46
00 26 48
00 26 51
00 26 51
00 26 51
00 26 52
00 26 53
00 26 54
00 26 54
00 26 58
00 26 58
00 26 59
00 27 01
00 27 01
00 27 05
00 27 05
00 27 06
00 27 06
00 27 07
00 27 08
00 27 08
00 27 09
00 27 10
00 27 12
00 27 13
00 27 13
00 27 13
00 27 14
00 27 15
δ2000
(deg m s)
18 54 28
18 55 20
18 55 56
18 57 09
18 54 43
18 57 00
18 51 49
18 54 51
18 58 19
18 56 01
18 51 56
18 55 24
18 56 33
18 54 37
18 59 10
18 57 47
18 55 12
19 02 23
18 52 24
18 53 45
18 56 08
18 59 51
18 56 57
18 51 16
18 54 16
18 55 50
18 55 51
18 58 18
19 03 10
18 52 21
18 57 08
18 59 36
19 01 59
19 00 48
18 55 39
18 51 35
18 54 08
19 02 45
18 56 24
18 59 12
V
(mag)
17.287
14.412
17.321
15.708
16.924
17.701
15.743
14.399
14.704
13.234
14.199
13.906
16.746
17.514
17.469
17.862
14.449
15.646
16.125
17.985
16.717
11.670
16.273
15.680
17.325
15.352
17.356
16.139
17.506
17.958
17.118
13.580
16.041
17.769
16.722
17.476
16.125
17.420
16.096
15.252
B−V
(mag)
0.750
0.876
0.813
1.097
0.642
0.703
1.043
0.653
0.787
1.072
0.669
0.717
0.900
0.606
1.481
0.760
1.072
0.885
0.740
0.580
0.617
0.640
1.149
1.075
0.484
0.618
0.652
0.626
1.054
0.662
0.777
0.912
1.151
1.463
1.139
0.842
0.914
1.427
0.832
1.055
V −R
(mag)
0.428
0.487
0.467
0.637
0.379
0.383
0.621
0.373
0.423
0.557
0.372
0.385
0.504
0.340
1.111
0.429
0.609
0.506
0.413
0.349
0.355
0.386
0.711
0.638
0.309
0.359
0.394
0.366
0.652
0.349
0.449
0.510
0.690
0.994
0.716
0.496
0.573
0.916
0.480
0.610
V −I
(mag)
0.765
0.873
0.908
1.151
0.703
0.746
1.090
0.706
0.802
1.044
0.683
0.767
1.016
0.694
2.426
0.887
1.111
0.942
0.774
0.668
0.725
0.758
1.360
1.142
0.666
0.726
0.810
0.744
1.174
0.735
0.878
0.961
1.226
2.024
1.297
0.946
1.068
1.686
0.938
1.145
N(B,V,R,I)
(9,9,17,9)
(9,10,16,10)
(9,10,17,9)
(9,10,17,9)
(9,10,17,9)
(9,10,17,10)
(9,10,17,9)
(9,10,16,10)
(9,10,16,10)
(9,9,7,3)
(9,10,16,10)
(9,10,16,10)
(9,10,17,10)
(9,10,17,10)
(9,10,17,10)
(9,10,17,10)
(9,10,17,9)
(9,10,17,10)
(9,9,17,9)
(8,9,15,9)
(9,10,17,10)
(3,4,3,3)
(9,10,17,10)
(9,10,17,10)
(9,10,17,9)
(9,10,17,10)
(9,10,17,10)
(9,10,17,10)
(7,7,11,5)
(9,9,17,10)
(9,10,17,10)
(9,10,16,10)
(9,10,17,10)
(9,10,17,10)
(9,10,17,10)
(9,9,17,10)
(9,10,17,10)
(8,9,15,9)
(9,8,14,9)
(5,6,14,8)
A single power law Fν ∝ ν −β was found to fit continuum of the observed spectra.
A chi-squared minimization for the power law yields an index of β = 0.59 ± 0.02 for
the averaged, E(B − V ) = 0.20 mag extinction corrected spectrum. This value becomes
1.07±0.06, if the spectra is corrected only for the Galactic extinction with E(B−V ) = 0.06
mag which then agrees fairly well with the value of β = 0.96 ± 0.03 derived by Matheson
et al. (2003).
Optical observations of GRB 021004 afterglow
7
Table 2. CCD BVRI broad band optical photometric observations of the GRB 021004
afterglow. At Hanle, 2-m HCT was used while at Nainital, 104-cm Sampurnanand optical
telescope was used.
Date (UT) of
2002 October
04.725000
04.728681
04.733333
04.741667
04.833333
04.834109
04.841667
04.850000
04.928472
04.929329
04.960544
04.982639
05.673588
05.809676
05.871713
04.691667
04.700000
04.713889
04.737870
04.784722
04.792361
04.800000
04.843495
04.856250
04.932640
04.944792
04.965278
04.969132
05.658657
05.680671
05.725729
05.823009
05.886007
06.730463
06.864549
Magnitude
(mag)
B− passband
18.37±0.01
18.43±0.01
18.43±0.01
18.52±0.01
18.87±0.01
18.88±0.02
18.95±0.01
18.96±0.01
19.19±0.02
19.21±0.02
19.31±0.02
19.33±0.02
20.33±0.02
20.53±0.02
20.62±0.02
V − passband
17.53±0.01
17.64±0.01
17.70±0.01
17.99±0.02
18.29±0.01
18.31±0.01
18.33±0.01
18.45±0.02
18.45±0.01
18.70±0.01
18.73±0.02
18.83±0.01
18.84±0.02
19.81±0.02
19.86±0.01
19.89±0.02
20.02±0.02
20.13±0.03
20.48±0.02
20.54±0.02
3.
Exposure time
(Seconds)
Telescope
600
600
600
600
600
600
600
600
300
600
600
300
1200
1200
1200
104-cm
HCT
104-cm
104-cm
104-cm
HCT
104-cm
104-cm
104-cm
HCT
HCT
104-cm
HCT
HCT
HCT
600
600
600
400
600
500
500
400
200
100
400
100
400
900
3×900
600
600
600
2×600
3×600
104-cm
104-cm
104-cm
HCT
104-cm
104-cm
104-cm
HCT
104-cm
104-cm
HCT
104-cm
HCT
HCT
104-cm
HCT
HCT
HCT
HCT
HCT
Date (UT) of
Magnitude
Exposure time
2002 October
(mag)
(Seconds)
V − passband (continued)
07.878831
20.80±0.02
3×600 + 400
08.818565
21.20±0.03
2×600
08.856076
21.21±0.03
2×600
10.778611
21.77±0.04
3×600
10.828819
21.66±0.03
3×600
10.881933
21.71±0.04
3×600
11.851632
21.91±0.03
4×600
I− passband
04.671528
16.48±0.01
300
04.679861
16.56±0.01
300
04.684722
16.58±0.01
300
04.745602
17.20±0.01
300
04.768750
17.31±0.01
300
04.773611
17.36±0.01
300
04.778472
17.32±0.01
300
04.850660
17.54±0.01
300
04.861806
17.48±0.02
100
04.936806
17.68±0.04
50
04.938032
17.77±0.02
300
04.976389
17.84±0.06
50
05.646030
18.83±0.02
600
05.712882
18.86±0.01
600
05.834213
19.11±0.02
600
05.895949
19.19±0.02
600
06.756343
19.51±0.02
600 + 300
06.775690
19.48±0.02
600 + 300
06.893345
19.62±0.03
2×500
07.908241
20.05±0.05
4×250 + 300
07.927083
19.85±0.11
6×300
08.675694
20.31±0.07
6×300
08.869317
20.35±0.05
400 + 300
08.884282
20.35±0.04
3×300
08.903009
20.45±0.05
3×300
09.829167
20.46±0.10
2×900
10.809030
21.00±0.14
2×900
11.760470
21.30±0.14
4×300
14.750690
21.64±0.18
3×900
Optical photometric light curves
We have used the present measurements in combination with the published data to study
the flux decay of GRB 021004 afterglow. Fig. 2 shows the plot of photometric measurements as a function of time. The X-axis is log (∆t = t − t0 ) where t is the time
of observation and t0 = 2002 October 4.504325 UT is the burst epoch. All times are
measured in unit of day.
Telescope
HCT
HCT
HCT
HCT
HCT
HCT
HCT
104-cm
104-cm
104-cm
HCT
104-cm
104-cm
104-cm
HCT
104-cm
104-cm
HCT
104-cm
HCT
HCT
HCT
HCT
HCT
HCT
HCT
HCT
104-cm
104-cm
HCT
HCT
HCT
104-cm
104-cm
104-cm
104-cm
8
Pandey et al.
Table 2. (Continued)
Date (UT) of
2002 October
04.625694
04.631944
04.637500
04.642361
04.654861
04.684051
04.690694
04.697257
04.703125
04.709213
04.715000
04.720972
04.749306
04.753438
04.754167
04.759028
04.763194
04.825417
04.856667
04.859028
04.920810
04.934722
04.940278
04.945139
04.954167
04.967361
05.616771
05.633576
05.641667
05.703252
05.736042
05.785718
05.793356
05.846794
05.855926
Magnitude
(mag)
R− passband
16.74±0.01
16.73±0.01
16.75±0.01
16.81±0.01
16.87±0.01
17.06±0.01
17.12±0.01
17.22±0.01
17.27±0.01
17.28±0.02
17.31±0.02
17.41±0.02
17.73±0.01
17.76±0.02
17.74±0.01
17.77±0.01
17.81±0.01
17.93±0.01
18.07±0.01
18.06±0.02
18.29±0.02
18.35±0.04
18.31±0.02
18.38±0.02
18.37±0.02
18.48±0.03
19.28±0.02
19.34±0.02
19.42±0.02
19.42±0.01
19.46±0.02
19.51±0.01
19.55±0.01
19.59±0.01
19.62±0.02
Exposure time
(Seconds)
Telescope
300
300
300
300
300
60
300
300
300
300
300
300
300
300
300
300
300
300
300
100
300
50
300
300
300
50
900
900
4×600
600
600
300
600
600
600
104-cm
104-cm
104-cm
104-cm
104-cm
HCT
HCT
HCT
HCT
HCT
HCT
HCT
104-cm
HCT
104-cm
104-cm
104-cm
HCT
HCT
104-cm
HCT
104-cm
104-cm
104-cm
104-cm
104-cm
HCT
HCT
104-cm
HCT
HCT
HCT
HCT
HCT
HCT
Date (UT) of
Magnitude
Exposure time
2002 October
(mag)
(Seconds)
R− passband (continued)
05.886806
19.73±0.02
3×900
05.908704
19.70±0.02
900
05.952072
19.72±0.02
900
06.648611
20.04±0.02
4×900
06.700972
19.98±0.02
3×600
06.807998
20.08±0.02
3×600
06.835880
20.09±0.01
3×600
06.926921
20.15±0.02
3×600
07.820856
20.38±0.02
4×400
07.887847
20.37±0.02
2×400 + 500
07.898611
20.50±0.09
5×300
07.948611
20.43±0.13
3×300
08.645139
20.69±0.04
6×300
08.769491
20.71±0.03
600 + 500
08.793299
20.72±0.03
2×600
09.727234
20.96±0.03
3×500
09.766319
20.99±0.02
3×600
09.800000
21.12±0.06
3×900
09.810069
20.95±0.02
4×600
10.706250
21.29±0.06
2×900
10.711505
21.23±0.03
2×500
10.732951
21.22±0.03
400 + 300
11.722220
21.55±0.11
2×900 + 2×300
11.799398
21.44±0.03
3×500
11.846667
21.43±0.03
3×500
13.675613
21.91±0.05
5×500
13.716991
21.93±0.05
6×400
13.756940
21.71±0.08
2×900
14.659720
21.87±0.11
2×900 + 1800
14.773877
22.04±0.07
5×500
14.815926
22.09±0.06
6×500
15.847570
22.22±0.07
4×600
15.852780
22.48±0.52
1×1800
16.688738
22.46±0.15
10×600
Table 3. Absorption lines in GRB 021004 afterglow spectrum.
Identification
Fe II
Fe II
Fe II
Fe II
Fe II
Mg II
Mg II
Fe II
Fe II
Mg II
Mg II
λ in (Å)
Observed
Rest
5577.8
2343.5
5649.3
2373.7
5669.2
2382.0
6185.0
2599.0
6194.6
2382.0
6655.3
2795.5
6672.7
2802.7
6733.1
2585.9
6762.7
2599.4
7273.1
2795.5
7292.9
2802.7
Redshift
1.380
1.380
1.380
1.379
1.601
1.381
1.381
1.604
1.602
1.602
1.602
Telescope
104-cm
HCT
HCT
104-cm
HCT
HCT
HCT
HCT
HCT
HCT
104-cm
104-cm
104-cm
HCT
HCT
HCT
HCT
104-cm
HCT
104-cm
HCT
HCT
104-cm
HCT
HCT
HCT
HCT
104-cm
104-cm
HCT
HCT
HCT
104-cm
HCT
Optical observations of GRB 021004 afterglow
9
Figure 1. Optical spectrum of the GRB 0021004 OA corrected for E(B − V ) = 0.20 mag in
the wavelength range 5500–9000 Å. The absorption lines are marked along with the estimated
redshift value.
3.1
Rapid variability in the BV RI optical light curve
The flux decay of most of the earlier GRB afterglows is generally well characterized by a
single power law F (t) ∝ (t − t0 )−α , where F (t) is the flux of the afterglow at time t and α
is the decay constant. However, optical light curves of GRB 021004 (Fig. 2) show rapid
variations with an overall flux decay especially during ∆t < 2 day. Among equally well
monitored GRB OA, GRB 021004 appears therefore peculiar. In order to see whether
variability is correlated in B, V, R and I passbands or not, we derived photometric colours
using optical and near-IR data and list them in Table 4. Where necessary, measurements
were interpolated between adjacent data points at one wavelength in order to determine
a contemporaneous value with another wavelength. There is no evidence for statistically
significant large variation in the photometric colours on these time scales. This result is
therefore contrary to the variability of spectroscopic colour reported by Matheson et al.
(2003).
In order to ensure that observed variability in the OA is not due to errors in photomet-
10
Pandey et al.
15.2
R mag of Henden (2002) comparison star
15.6
16.0
(I-1.2)
18.0
R
20.0
(V+1.0)
(B+2.0)
22.0
24.0
BVRI Light-curve of GRB 021004 afterglow
26.0
0.4
0.0
-0.4
-2.0
-1.5
-1.0
-0.5
0.0
0.5
Log (t - 2002 Oct 4.5043) in day
1.0
1.5
Figure 2. Light curves of GRB 0021004 afterglow in optical photometric BV RI passbands are
shown in the middle panel. Marked vertical offsets have been applied to avoid overlapping of
data points of different passbands. For comparison, R magnitude of Henden (2002) comparison
star is also plotted in the upper panel. The BV RI band residuals in the sense observed minus
power-law fitted magnitudes are displayed in the lower panel.
ric measurements, we also plotted in Fig. 2, the R value of Henden’s (2002) comparison
star against time which showed no variability while Fig. 2 clearly indicates peculiar behaviour of the light curve showing achromatic variability in BV RI passbands during early
phase. A large fraction of these observations have been carried out using the 1-m class
optical telescopes. This indicates that in future large amount of observing time is available on these telescopes (cf. Sagar 2000), will play an important role in understanding
the origin of such short term variability in the light curves of GRBs during early times.
In order to analyse the rapid flux variations, we plot in the bottom panel of Fig.
11
Optical observations of GRB 021004 afterglow
Table 4. Broad band photometric colours of GRB 021004 OA at selected epochs.
∆t
(in days)
0.22
0.34
0.46
1.17
1.37
1.66
2.01
2.98
5.57
7.73
12.46
(B − V )
(mag)
0.52±0.03
0.53±0.03
0.51±0.03
0.49±0.03
0.52±0.04
0.57±0.05
0.60±0.06
0.65±0.10
0.63±0.10
0.58±0.10
0.58±0.10
(V − I)
(mag)
1.05±0.03
1.03±0.03
0.99±0.04
0.95±0.06
0.93±0.08
1.00±0.06
0.98±0.10
1.01±0.10
0.75±0.20
0.83±0.15
(B − J)
(mag)
(B − H)
(mag)
2.2±0.15
2.3±0.15
(B − K)
(mag)
4.2±0.3
3.7±0.3
3.3±0.15
3.2±0.15
3.4±0.15
2, residuals after subtracting the best fitted power law values from the corresponding
observed ones against time. The variations appear to be achromatic and are clearly
visible due to dense temporal coverage of photometric observations. They have a mean of
0.02±0.12, 0.07±0.15, 0.13±0.18 and 0.05±0.15 mag in B,V ,R and I filters respectively.
We obtain a rough estimate of these time variations by fitting gaussian, which gives the
FWHM values to be ∼ 11.5 and 21 hour for the bumps between ∆t = 0.25 to 1 day
and 1.1 to 2.1 day respectively. These periods are considerably larger than 0.7 hour
period found by Holland et al. (2002) and Jakobsson et al. (2003) for GRB 011211. The
peculiar nature of the GRB 021004 OA can be explained in terms of variable external
density of the medium or variation in energy of the blast wave with time (Nakar, Piran
& Granot 2003). Lazzati el al. (2002) have also explained this peculiarity in terms of
density enhancements of the surrounding medium.
3.2
Parameters from optical light curves
In the light curves there is steepening after ∆t > 6 day appears to be present in B,V ,R
and I passbands. Achromatic fluctuations are also clear in BV RI passbands. In order to
determine the flux decay and related parameters from the optical light curves following
analyses have been carried out.
1. The earliest 3 data points in R passband show a flux decay with α = 0.69 ± 0.05.
This could be due to reverse shock emissions as the value of α is generally around
1.0 during early time flux decay of forward shock emissions (cf. Kobayashi & Zhang
2003 and section 4).
2. It is unlikely that emission from reverse shock will contribute significantly after
12
Pandey et al.
∆t > 0.1 day and the steepening in the light curve appears to be 6 days after the
burst. We have therefore determined flux decay constant for the OA using least
square linear fit to the data points of ∆t < 5 day and found values of 0.92±0.08,
0.93±0.03 and 1.13±0.04 in V ,R and I passbands respectively. Average value of
the early time flux decay constant of the OA is therefore 0.99±0.05. This is in good
agreement with the values of early time flux decay constants of well observed GRB
OA and also is as expected theoretically.
3. To determine the late time flux decay constant and break time, we fitted the following empirical function (see Rhoads & Fruchter 2000) which represents a broken
power-law in the light curve in presence of underlying host galaxy.
m = mb +
2.5
[log10 {(t/tb )α1 s + (t/tb )α2 s } − log10 (2)] + mg
s
where α1 and α2 are asymptotic power-law slopes at early and late times with
α1 < α2 and s > 0 controls the sharpness of the break, with larger s implying a
sharper break. mb is the magnitude at the cross-over time tb . mg is the magnitude
of underlying host galaxy. The function describes a light curve falling as t−α1 at
t << tb and t−α2 at t >> tb . In jet models, an achromatic break in the light
curve is expected when the jet makes the transition to sideways expansion after the
relativistic Lorentz factor drops below the inverse of the opening angle of the initial
beam. As there are rapid variations around overall early time flux decay, we fitted
the above function in BV RI bands, for ∆t > 2 day to determine the parameters
of the jet model. In order to avoid a fairly wide range of model parameters for a
comparable χ2 due to degeneracy between tb , mb , mg , α1 , α2 and s, we have used
fixed values of α1 = 0.99 and s in our analyses and find that the minimum value
of χ2 is achieved around s = 4. We also fixed the value of mg for a minimum
value of χ2 for different filters. The fitted values of host galaxy contributions mg
are ∼ 24.79, 24.59, 24.35 and 23.79 mag for B,V ,R and I passbands respectively.
The least square fit values of the parameters tb , mb , and α2 are 6.51±0.12 day,
21.37±0.03 mag, and 2.06±0.05 respectively in R band, with a corresponding χ2 of
2.44 per degree of freedom (DOF ). For V passband fitted values of tb , mb , and α2
are 6.44±0.28 day, 21.79±0.06 mag, and 1.96±0.17 respectively with χ2 3.16 per
DOF . For B and I filters we also fixed the value of tb at 6.5 day to determine the
values of α2 . The values of α2 are 2.07±0.40 and 1.78±0.15 respectively for B and
I filters with mb values of 22.46±0.03 and 20.87±0.01 mag. For B and I filters χ2
values are 1.05 and 2.73 per DOF . This indicates that the observed break in the
light curve is sharp, unlike the smooth break observed in the optical light curve of
GRB 990510 (cf. Stanek et al. 1999; Harrison et al. 1999) but similar to the sharp
break observed in the optical light curves of GRB 000301c (cf. Berger et al. 2000,
Sagar et al. 2000, Pandey et al. 2001); GRB 000926 (cf. Harrison et al. 2001,
Sagar et al. 2001a, Pandey et al. 2001); GRB 010222 (cf. Masetti et al. 2001;
Sagar et al. 2001b; Stanek et al. 2001; Cowsik et al. 2001) and GRB 011211 (cf.
Jakobsson et al. 2003). In Fig. 2 the best fit light curves obtained in this way for
Optical observations of GRB 021004 afterglow
13
BV RI passbands are shown. It can also be seen that our own observations follow
the fitted curves very well and fill gaps in the published data.
In the light of above, we conclude that the parameters derived from the optical
BV RI light curves are α = 0.69±0.05 for reverse shock emission and tb = 6.5±0.2 day,
α1 = 0.99±0.05 and α2 = 2.0±0.2 for the OA. These parameters are improved further in
the next section by fitting the multi-wavelength observations with the standard fireball
model of GRBs.
4.
Modelling of the GRB 021004 afterglow
We attempt modelling the behaviour of GRB 021004 OA along the lines of standard GRB
model proposed by Kobayashi & Zhang (2003).
The 7-400 keV gamma-ray fluence (Lamb et al. 2002) implies an isotropic-equivalent
energy of 4.6 × 1052 erg emitted in the burst radiation, for H0 = 65 km/s/Mpc in a
Ωm = 0.3, ΩΛ = 0.7 cosmological model. The corresponding comoving passband of
23 − 1330 keV contains the bulk of the emitted energy in most GRBs, and a k-corrected
estimate of Bolometeric energy is unlikely to exceed this by more than ∼ 50% (Bloom
et al 2001). Assuming a similar amount of energy to remain in the fireball to power
the afterglow (Piran et al. 2001), one finds that for a typical ǫe ∼ 0.1 and ǫB ∼ 10−2
(Panaitescu & Kumar 2001) the frequency νm of maximum radiation in the afterglow
spectrum should lie close to the optical band at ∼ 0.1 day after the burst (Sari, Piran &
Narayan 1998; Kobayashi & Zhang 2003). The brightening of GRB 021004 OA optical
lightcurve at ∼ 0.1 day, relative to the extrapolated early decay, could therefore be
attributed to the passage of νm through the optical band. The three early R-band
observations (Fox 2002) which show a power-law decay of α = 0.69±0.05 before the
brightening can then be understood in terms of a decaying prompt emission from the
reverse shock (Kobayashi & Zhang 2003). We exclude this early emission from our further
discussion and restrict ourselves to the properties of the forward shock emission.
In Fig. 3 we compare the predictions of a standard afterglow model with electron
energy distribution power-law index p=2.27 and a jet-break time tb of 6.7 days. For this,
the observed magnitudes/fluxes have been corrected for standard Galactic extinction law
given by Mathis (1990) and the effective wavelength and normalization by Fukugita et
al. (1995) for U, B, V, R, I and by Bessell & Brett (1988) for near-IR have been used.
The fluxes thus derived are accurate to about 10% in optical and about 25% in near-IR.
For the model parameters mentioned above, relative normalization of the light curves
in different optical passbands become consistent with the data if the total extinction is
E(B − V ) = 0.20 mag, which has been used in our analysis. The Galactic extinction
in this direction is estimated to be E(B − V ) = 0.06 mag from the smoothed reddening
map provided by Schlegel, Finkbeiner & Davis (1998). The additional extinction may
then be attributed to small scale fluctuations in the distribution of dust in our galaxy, or
14
Pandey et al.
Figure 3. Multi-band observed light curves (left panel) and broadband spectrum (right panel)
of the GRB 021004 OA are compared with model predictions shown as solid curves. A total
extinction of E(B − V )=0.20 mag has been used. For clarity of display, in the left panel the
B,V ,I and radio light curves are shifted vertically by −1.0, −0.5, +0.5 and +1.0 respectively
in logarithmic scale. The radio light curve at 10 GHz is constructed, by extrapolation with
expected spectral slope, from measurements reported at 22.5 GHz, 15 GHz and 8.46 GHz at
different epochs (Frail & Berger 2002, Pooley et al. 2002, Berger et al. 2002). The frequency of
10 GHz was chosen to correspond with a similar plot presented by Kobayashi & Zhang (2003).
The model uses νa = 2.1 GHz, νm = 2.5 × 1014 Hz and νc = 3.3 × 1016 Hz at t = 0.06 day; a jet
break time tb = 6.7 day and an electron energy distribution index p = 2.27. The model includes
the host galaxy contribution estimated in section 3.2. In the right panel an expected spectrum
with p = 2.27 is shown with the Chandra HETG measurement (Sako & Harrison 2002a) and
optical and near-IR observations at the same epoch, ∼ 1.37 day. A similar spectrum is also shown
at t = 5.67 day, the epoch of the cm-wave radio observations reported by Berger et al. (2002).
Where necessary, fluxes measured at optical, near-IR and radio wavelengths were interpolated
between adjacent data points at one wavelength in order to determine a contemporaneous value
with another wavelength.
Optical observations of GRB 021004 afterglow
15
may, in part, originate even in the host galaxy of the GRB. The spectral slope deduced in
section 2.2 from the HCT low resolution spectrum is also consistent with p = 2.27 once
the above total extinction is taken into account. The flux decay constants derived in the
last section are also consistent with the parameters used in the model.
Given the above model parameters we find that the broadband behaviour of the OA
is well explained. However, such a model cannot reproduce short term variations, as
seen during the interval ∼ 0.5 − 2 days in the optical light curves (see Fig. 2). The
reason for these short term variations could lie in density variations in the circum-burst
medium, as conjectured by Lazzati et al. (2002) and Nakar et al. (2003). Fig. 3 shows the
broadband spectrum including X−ray, optical-near IR and radio observations and the
model predictions. The cooling break νc is located between the optical and X−ray bands.
The spectrum observed within the X−ray band at ∼ 1.37 day (Sako & Harrison 2002a)
is Fν ∝ ν −1.1±0.1 , and the decay rate is t−1.0±0.2 . Both are consistent with p = 2.27 and
ν > νc , at a time before the jet break. Our model predictions are also in good agreement
with the Chandra observation at t ∼ 52 day (Sako & Harrison 2002b). The presence of
a jet break between the two observations results in the apparent temporal decay slope of
∼ 1.7 reported by Sako & Harrison (2002b).
The 1.4 GHz to 8.5 GHz radio spectrum at 5.67 days (Berger et al. 2002) is well fit by
the model, assuming a self-absorption frequency νa near 2 GHz. The same assumptions
lead to a good explanation of the cm-wave radio light curve. Fig. 3 shows the derived
light curve at 10 GHz from measurements reported at nearby frequencies (Frail & Berger
2002, Pooley et al. 2002, Berger et al. 2002) and the model prediction. However, this
model is unable to reproduce the 85 GHz flux of 2.5 mJy observed at ∆t = 1.45 days
(Bremer & Castro-Tirado 2002). Although at the maximum of the spectrum (νm ) the
flux rises to 2.75 mJy in the model, at ∆t = 1.45 days, νm is located well above 85 GHz
and the predicted flux is only ∼ 1 mJy at 85 GHz. It may well be that a part of the
emission observed at 85 GHz comes from the host galaxy as in the case of GRB 010222
(Frail et al. 2002) which should be seen to remain visible after the afterglow fades away.
5.
Discussions and Conclusions
We present the broad band BV RI photometric and low-resolution spectroscopic optical
observations of the OA associated with GRB 021004 starting about 3 hour after the
burst. Our last photometric observations are at about ∆t = 12 days. These observations
in combination with the published multi-wavelength data have been used to study the
flux decay and to derive parameters of the GRB and its afterglow. We have used secure
photometric calibrations in the present analyses. The optical observations obtained by
Fox (2002) during the first 20 minutes of the burst indicate that GRB 021004 is the
second GRB OA after GRB 990123 from which optical emission from the reverse shock
has been observed (Galama et al. 1999). The dense temporal BV RI passband light
curve indicates rapid light variations. Such flux variations from a power-law decay have
16
Pandey et al.
been reported only for GRB 000301c (Sagar et al. 2000a, Masetti et al. 2000, Jensen
et al. 2001, Garnavich, Loab & Stanek 2000, Gaudi et al. 2001) and for GRB 011211
(Holland et al. 2002, Jakobsson et al. 2003) so far. However, the amplitude of oscillation
is maximum in the case of GRB 021004 OA being ∼ 0.5 mag. The light curves show a
steepening superposed on the achromatic, rapid variations which could be detected mainly
due to the dense observations in BV RI filters. This indicates that in future the small
telescopes, as large amount of observing time is available on them (cf. Sagar 2000), will
play an important role in understanding the origin of such short term variability in the
light curves of GRBs during early times. The overall flux decay in observed light curves is
well understood in terms of a jet model. The flux decay constants at early and late times
derived from least square fits to the light curves are 0.99±0.05 and 2.0±0.2 respectively.
The value of the jet break time is about 7 day. The total extinction in the direction of the
OA is E(B − V ) = 0.2 mag. The low-resolution spectrum corrected for this extinction
yields a spectral slope of β = 0.6±0.02. The photometric colour distributions determined
in optical and near-IR regions for various epochs indicate that spectral index of the GRB
021004 afterglow has not changed significantly during a period of about 15 days after
the burst, while the flux decay slope has steepened from 1.0 to 2.0. GRB 021004 thus
becomes one more burst for which a clear achromatic break in the light curve is observed.
This is generally accepted as an evidence for collimation of the relativistic GRB ejecta
in accordance with the prediction by recent theoretical models (Mészáros & Rees 1999;
Rhoads 1999; Sari Piran & Halpern 1999).
Recent afterglow observations of GRBs show that a relativistic blast wave, in which
the highly relativistic electrons radiate via synchrotron mechanism, provides a generally
good description of the observed properties. In the case of GRB 021004 OA also, it
appears that a standard fireball afterglow model, with a combination of emission from a
forward and a reverse shock can account for most of the overall behaviour of the afterglow.
The observed fluxes, however, show unexplained fluctuations, falling significantly below
model predictions in the range ∆t = 0.5–2 days. Density variations in the circum-burst
medium is one possible explanation of this behaviour (Wang & Loeb 2000, Lazzati et
al. 2002, Nakar et al. 2003, Heyl and Perna 2003). While an alternative explanation in
terms of micro-lensing (Garnavich, Loeb & Stanek 2000) cannot be entirely ruled out, the
multiple bumps seen in this light curve would not be natural in this model. The observed
jet break time of ∼ 7 day, along with the burst fluence, leads to an estimate of the jet
opening angle of ∼ 7◦ (for an assumed γ−ray efficiency ηγ = 0.2 (Frail et al. 2001), and a
circumburst density n = 0.3 cm−3 inferred from reverse-shock modelling by Kobayashi &
Zhang (2003), similar to the opening angles inferred in other jetted afterglows (see, e.g.
Panaitescu & Kumar 2001). The inferred opening angle implies a total emitted gammaray energy of Eγ ∼ 3.5 × 1050 erg, close to the peak of the Eγ distribution in GRBs as
shown by Frail et al. (2001). The modelling of the radio emission suggests that excess
emission might have been detected at 85 GHz, possibly due to the emission from the host
galaxy. If this turns out to be true, then the observed emission would indicate a strong
star formation activity in the host galaxy. The multiple blue shifted H, C-IV and Si-IV
absorption lines in the spectrum of GRB021004 OA, with a velocity span of 3200 km/s,
Optical observations of GRB 021004 afterglow
17
could be interpreted as a clumpy WC star wind environment (Mirabal et al. 2002b).
However our modelling indicates that the light curve after ∼ 0.1 day is better explained
by a circumburst medium of nearly uniform density with small scale density variations
rather than a r−2 wind density profile. This might imply a variable mass loss rate in
the wind (Heyl and Perna 2003) or that the circumburst medium is composed not of
Wolf-Rayet wind but of expanding ejecta of a supernova preceding the burst (Salamanca
et al 2002, Wang et al 2003). In either case, these observations provide a strong support
in favour of collapsar origin of this burst in particular, and of long duration GRBs in
general.
The peculiarity in the light curves of GRB 021004 could be noticed mainly due to
dense as well as multi-wavelength observations during early times. Such observations of
recent GRBs have started revealing features which require explanation other than generally accepted so far indicating that there may be yet new surprises in GRB afterglows.
Acknowledgements
We thank an anonymous referee for comments which helped us improve the paper. This
research has made use of data obtained through the High Energy Astrophysics Science
Archive Research Center Online Service, provided by the NASA/Goddard Space Flight
Center. L. Resmi is supported by a CSIR research fellowship.
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